Internal combustion engines combust an air and fuel mixture within cylinders to reciprocally drive pistons. The pistons rotatably drive a crankshaft to provide drive torque to a powertrain. Exhaust generated by the combustion process is exhausted from the engine through an exhaust manifold and treated by an exhaust system.
Engine systems often include an exhaust gas recirculation (EGR) system to reduce engine emissions. EGR systems re-circulate exhaust gases back into the cylinders, which tends to limit the amount of oxygen available for combustion. Limiting the oxygen available for combustion lowers combustion temperatures and reduces engine emissions. EGR systems can also improve fuel economy and/or performance when spark timing and fuel injection are optimized along with the operation of the EGR system. Debris build-up within the EGR system restricts the flow of exhaust and minimizes the effectiveness of the EGR system. Thus, an EGR diagnostic test may be performed to determine when EGR flow is restricted.
The EGR diagnostic test compares a maximum manifold absolute pressure (MAP) when the EGR valve is open to a maximum MAP when the EGR valve is closed. The diagnostic test uses the maximum MAP difference as an indication of EGR flow. This method requires positioning the EGR valve in open and closed positions.
During operation of the engine, operating characteristics of the EGR valve are affected by temperature changes. Each time the EGR diagnostic test is performed, the EGR valve must locate a new target position. The new target position is determined by starting from an initial position and working towards a position that allows a desired flow through the EGR valve. Attempting to locate the new target position can create excessive variations in EGR flow, which increases emissions. Furthermore, the current positioning method is difficult to calibrate due to the differing EGR valve characteristics during operation of the engine.